Method for powertrain operation

ABSTRACT

A method for starting and operating an engine of a mild hybrid vehicle following start-up is disclosed for an engine having low and high voltage starting devices, and controlled by an electronic controller. The method comprises using the high voltage starting device as a motor to crank the engine whenever possible so as to partially discharge a high voltage battery. The method further comprises using the high voltage starting device as a generator after starting the engine if the exhaust gas temperature needs to be increased so as to assist with light-off of one or more exhaust gas after treatment devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims foreign priority benefits under 35 U.S.C.§119(a)-(d) to GB 1401158.9, filed Feb. 12, 2014, GB 1417955.0, filedOct. 10, 2014, and GB 1417956.8, filed Oct. 10, 2014, which are herebyincorporated by reference in their entirety.

TECHNICAL FIELD

Subject matter disclosed herein relates to an internal combustionengine, and in particular to a method and apparatus for improvedoperation of such an engine.

BACKGROUND

It is well known to use a low voltage (12 volt) starter motor to startan internal combustion engine even when the engine is at a very lowtemperature such as for example minus 30° C. It is further known toprovide an engine with a high voltage (for example 48 volt) beltintegrated starter generator (BISG) that is driveably connected to acrankshaft of the engine by a drive belt and is able to be either drivenby the engine to generate electrical power or drive the engine either toassist with power output or for starting the engine.

Normally the BISG is only used when the temperature of the engine isabove a relatively warm temperature corresponding to a low end of anormal temperature operating range of the engine such as, for example60° C., because a conventional BISG will normally have insufficienttorque to start the engine at lower temperatures.

It is further known that the fuel saving ability of a vehicle isrestricted or limited by the need to first light-off any exhaustaftertreatment devices. It is therefore desirable, particularly when theengine is cold, to apply a load to the engine after it has started inorder to increase the temperature of the exhaust gas and thereby reducethe time taken to light-off any attached exhaust aftertreatment devices.

The BISG can be usefully employed to apply such a heating load but isoften not able to do so because the current state of charge (SOC) of theassociated high voltage battery will not permit the BISG to be run as agenerator.

The inventors have realized that if the BISG is used over a largertemperature range to start the engine then the discharging effect ofusing the BISG to start the engine can beneficially be used to reducethe state of charge of the high voltage battery thereby allowing theBISG to be subsequently operated as a generator after the engine hasstarted to apply a load to the engine and speed up light-off of theexhaust aftertreatment devices.

In addition, in the case of a mild hybrid vehicle, stop-start operationdoes not have to be restricted until the engine temperature has reachedthe low end of the normal temperature operating range of the engine (60to 120° C.) but can be used whenever BISG starting is possible therebyincreasing the opportunities for stop-start operation.

SUMMARY

There is provided a method for improved engine operation comprisingstarting an engine of a motor vehicle having a high voltage beltintegrated starter-generator and a low voltage starter motor using thehigh voltage belt integrated starter-generator whenever it is able toeffectively crank the engine and, if after starting of the engine, it isrequired to light-off one or more exhaust aftertreatment devices, usingthe high voltage belt integrated starter-generator as a generator so asto load the engine to increase the temperature of the exhaust gas fromthe engine.

The method may further comprise using the high voltage belt integratedstarter-generator as a generator to increase the temperature of theexhaust gas of the engine until it is no longer required to furtherincrease the temperature of the exhaust gas.

The method may further comprise using the high voltage belt integratedstarter-generator as a generator only if the state of charge of abattery charged by the high voltage belt integrated starter-generator isbelow a predefined upper charging limit.

If after starting of the engine and provided there is no requirement tolight-off one or more exhaust aftertreatment devices, the method mayfurther comprise determining whether torque assist of the engine isrequired and, if torque assist is required, using the high voltage beltintegrated starter-generator as a motor to provide torque assist to theengine.

The method may further comprise using the high voltage belt integratedstarter-generator for torque assist only if the state of charge of abattery providing power to the high voltage belt integratedstarter-generator to operate it as a motor is above a minimum permittedcharge level.

The high voltage belt integrated starter-generator may be able toeffectively crank the engine unless the temperature of the engine isbelow a predefined low temperature limit below which the engine isstarted using the low voltage starter motor.

The low temperature limit may be a temperature well below the normaloperating range of the engine.

The low temperature limit may be a temperature below zero degreesCelsius.

The low temperature limit may be a temperature falling within a range ofminus 40 to plus 5 degrees Celsius.

The high voltage belt integrated starter-generator may be able toeffectively crank the engine if it is able to crank the engine at aspeed greater than a defined rotational speed.

The high voltage belt integrated starter-generator may be able toeffectively crank the engine if it is able to produce an acceleration ofthe engine greater than a defined acceleration.

The high voltage belt integrated starter-generator may be able toeffectively crank the engine if the state of charge of a battery used topower the high voltage belt integrated starter-generator is above apredefined level before cranking commences.

There is provided a mild hybrid motor vehicle having an apparatus forimproved engine operation comprising a high voltage belt integratedstarter-generator, a low voltage starter motor and an electroniccontroller to control the operation of at least the high voltage beltintegrated starter generator and the low voltage starter motor whereinthe electronic controller is operable use the high voltage beltintegrated starter-generator to crank the engine whenever it is able toeffectively crank the engine and, if after starting of the engine, it isrequired to light-off one or more exhaust aftertreatment devices of themotor vehicle, using the high voltage belt integrated starter-generatoras a generator so as to load the engine to increase the temperature ofthe exhaust gas from the engine.

The high voltage belt integrated starter-generator may be able toeffectively crank the engine unless the temperature of the engine isbelow a predefined low temperature limit below which the engine isstarted using the low voltage starter motor.

The low temperature limit may be a temperature well below the normaloperating range of the engine.

The low temperature limit may be a temperature below zero degreesCelsius.

The low temperature limit may be a temperature falling within a range ofminus 40 to plus 5 degrees Celsius.

The high voltage belt integrated starter-generator may be able toeffectively crank the engine if it is able to crank the engine at aspeed greater than a defined rotational speed.

The high voltage belt integrated starter-generator may be able toeffectively crank the engine if it is able to produce an acceleration ofthe engine greater than a defined acceleration.

The high voltage belt integrated starter-generator may be able toeffectively crank the engine if the state of charge of a battery used topower the high voltage belt integrated starter-generator is above apredefined level before cranking commences.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a mild hybrid motor vehicle including anapparatus for starting and operating an engine of the motor vehicle;

FIG. 2 is a flow chart of a method for starting an engine forming afirst part of a method for improved engine operation; and

FIG. 3 is a flow chart forming a second part of the method for improvedengine operation.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

With reference to FIG. 1 there is shown a mild hybrid vehicle 5 havingan engine system including an engine 11 drivingly connected to a gearbox12 and an apparatus for starting the engine 11. One or more exhaust gasaftertreatment devices 6 are arranged to receive exhaust gas from theengine 11.

The apparatus for starting the engine 11 comprises an electroniccontroller in the form of a control unit 10, a low voltage startersystem including a starter motor 13, a low voltage battery 17 and a lowvoltage battery management system 15 and a high voltage starter systemincluding a belt integrated starter-generator 14, a high voltage battery18 and a high voltage battery management system 16.

The apparatus for starting the engine 11 further comprises a DC to DCvoltage converter for selectively connecting the high voltage battery 18to the low voltage battery 17 for the purpose of recharging the lowvoltage battery 17 and a number of inputs 20 for providing informationto the electronic controller 10.

A ‘mild hybrid vehicle’ is a vehicle having an electric motor/generator(starter-generator) driveably connected to an engine of the vehicle to

-   a/ assist the engine of the vehicle by producing mechanical torque    using electricity stored in a high voltage battery (torque assist);-   b/ capture energy from the vehicle with no fuel penalty;-   c/ store captured energy as electricity in the high voltage battery;-   d/ start the combustion engine of the vehicle; and-   e/ provide electrical energy to users of the vehicle.-   Such vehicles are sometimes alternatively referred to as    ‘micro-hybrid vehicles’.

The electric motor is not used on its own in a mild hybrid vehicle todrive the vehicle it is only used to start the engine or assist theengine in driving the vehicle so as to reduce the instantaneous fuelconsumption of the engine.

Therefore the BISG 14 can operate in two modes, in the first mode it isdriven by the engine 11 to produce electrical power for storage in thehigh voltage battery 18 (HV battery) and in the second mode is producestorque to either supplement the torque produced by the engine 11 or foruse in starting the engine 11.

The electronic controller is described in this case as being a singlecontrol unit 10 operable to control not only the general operation ofthe engine 11 but also the low and high voltage starter systems. It willhowever be appreciated that the electronic controller could comprise ofa number of interlinked electronic controllers providing in combinationthe same functionality.

In the case of the example shown in FIG. 1 the inputs to the electroniccontroller 10 include at least one input from which the temperature ofthe engine 11 can be deduced and in this case comprise sensor inputsindicative of ambient air temperature; intake air temperature; coolanttemperature; cylinder head temperature and engine cylinder blocktemperature and at least one input indicative of engine speed and/orBISG speed.

The high voltage battery 18 of the high voltage starter system isoperatively connected via the DC to DC converter 19 to the battery 17 ofthe low voltage starter system so that the low voltage battery can berecharged by the BISG 14 when required.

The electronic controller 10 is operatively connected to the DC to DCconverter 19, to the high and low voltage battery management systems 16and 15, the starter motor 13 and BISG 14 and various other devices andsensors associated with the engine 11.

The electronic controller 10 could also include an engine stop-startcontroller for the motor vehicle 5 and the inputs 20 would then alsoinclude inputs for use in determining when the engine 11 should beautomatically stopped in order to save fuel. Such a stop is referred toherein as an ‘E-stop’ because its function is to increase the economy ofthe engine 11. As is well known in the art various triggers can be usedto initiate an E-stop based upon operation of various driver actions andfurther triggers based upon driver actions can be used to initiate anautomatic restart following an E-stop. Any suitable combination of stopand start triggers can be used. An automatic engine stop or E-stop isone where the engine 11 is temporarily stopped to save fuel and reduceemissions by the electronic controller 10 in response to one or moreconditions based upon driver actions.

In the case of the example being described ‘low voltage’ is a voltage ofcirca 12 volts and ‘high voltage’ is a voltage of circa 48 volts. Itwill however be appreciated that the embodiments contemplated herein arenot limited to these voltages although they are advantageous in thatequipment utilizing such voltages is readily available.

Operation of the apparatus for starting the engine 11 is as follows.

When the controller 10 receives an input indicative that the engine 11is to be started it has to decide whether to use the starter motor 13 toperform the start or the BISG 14. The input indicative that the engineis required to be started could be a manual one resulting from a driveractuation of a starting input device such as a start button or ignitionkey or can be produced automatically at the end of an “E” stop.

The default condition for the electronic controller 10 is to use theBISG 14 to start the engine 11 because this will result in a higherstart quality and also has the effect of almost instantaneously reducingthe state of charge (SOC) of the high voltage battery 18 due to the highcurrent draw required to start the engine 11. Reducing the SOC of thehigh voltage battery 18 is particularly advantageous if the engine 11 isnot up to temperature because it is then desirable to light-off anyexhaust aftertreatment devices as soon as possible in order to reduceexhaust emissions and permit the engine control strategy to be morefocused on CO2 reduction.

By using the BISG 14 to start the engine 11 it is ensured thatimmediately after the engine 11 has started the BISG 14 can be switchedto the first or charging mode to recharge the high voltage battery 18thereby applying an additional load to the engine 11 which speeds uplight-off of any exhaust aftertreatment devices by increasing thetemperature of the exhaust gas exiting the engine 11 (exhaust gasheating).

There are several situations where the starter motor 13 is used to startthe engine 11 instead of the BISG 14.

The first of these is when the engine 11 is sensed to be at a very lowtemperature well below the normal operating temperature of the engine.At low temperatures the torque required to start the engine 11 increasesdramatically, particularly if ice crystals form within the engine 11.Therefore, if the engine temperature is below a minimum temperature(T_(min)), the starter motor 13 is used to start the engine 11. It willbe appreciated that a starter motor 13 is geared so as to produce alarge output torque but can only rotate the engine at a relatively lowspeed. Typically a maximum cranking speed produced by a starter motorwill be circa 400 RPM.

The minimum temperature T_(min) may be derived from experimental workfor each engine/BISG combination but in all cases is the temperaturebelow which the torque available from the BISG 14 is unlikely to besufficient to produce a clean or good quality start. It will beappreciated that there is a compromise between increasing the torqueoutput capacity of the BISG 14 versus its general operating efficiencyand that, at low temperatures, other factors such as drive beltefficiency and durability have to be taken into account. The value ofthis minimum temperature T_(min) will however depend upon the size andtype of the engine and the torque capacity of the BISG. For example inthe case of a small petrol engine it may be possible to use the BISG toproduce a clean engine start down to minus 40° C. (T_(min)=−40° C.)whereas, in the case of a large diesel engine or large petrol engine,the BISG may not be able to cleanly start the engine below plus 5° C.(T_(min)=+5° C.). Therefore T_(min) is generally within the range of−40° C. to +5° C. but generally is likely to be a temperature below 0°C. in most cases depending upon the above referred to factors and in allcase is well below the normal temperature operating range of the engine11.

A second situation where the starter motor 13 is used instead of theBISG 14 is when the cranking speed or acceleration of the engine 11produced by the BISG 14 is unacceptably low.

In its most simplistic form engine speed can be checked by using acomparison of the current cranking speed Nc with a defined speed limitNc_(min) and if the current measure of cranking speed Nc is below thevalue set for Nc_(min) reverting to the starter motor 13 to start theengine 11. For example by checking the speed of the engine 11 apredefined period of time after cranking has commenced with an expectedengine speed it can be determined if the BISG 14 is able to effectivelycrank the engine 11. The value of Nc_(min) can be a variable based upontemperature and/or time since cranking commenced in which case a look upchart or table referencing time/temperature and engine speed could beused to check the predicted ability of the BISG 14 to start the engine11.

A similar approach could be used if the test is based upon engineacceleration. The engine speed in this case being used to produce avalue of engine acceleration which is compared to a predefined orexpected rate of acceleration if the BISG is functioning normally andwould be expected to produce a good quality start. As before theexpected rate of acceleration could be varied based upon enginetemperature and a look up chart or table referencing temperature andengine acceleration could be used to check the predicted ability of theBISG 14 to start the engine 11.

There are various reasons why the cranking speed or rate of engineacceleration could be unacceptably low, for example and withoutlimitation, the resistance to cranking of the engine 11 could be higherthan expected, the SOC of the high voltage battery 18 could be low,there could be a fault in the BISG 14, there could be insufficienttorque transfer capacity due to drive belt slip or the high voltagebattery management system 16 could block the drawing of current from thehigh voltage battery because the SOC of the high voltage battery is ator below a lower limit.

A third situation where the starter motor 13 is used instead of the BISG14 is when the state of charge of a battery used to power the highvoltage belt integrated starter-generator is below a predefined levelbefore cranking commences. It will be appreciated that there has to be acertain level of charge in the high voltage battery 18 for it to be ableto successfully power the BISG 14 during an engine start. In additionthis predefined level of charge will vary depending upon the expectedtorque required to start the engine 11. Therefore, if the state ofcharge of the high voltage battery 18 is below the predefined level forthe current temperature, it is likely that the BISG 14 will not be ableto effectively crank the engine 11 and so the starter motor 13 is usedto start the engine 11. However, in most cases the state of charge ofthe high voltage battery 18 will be sufficient and so this situation isunlikely to arise very often.

Therefore whenever cranking using the BISG 14 is determined to beunavailable or ineffective the electronic controller 10 is operable touse the starter motor 13 to start the engine 11 even if the temperatureis above the low temperature limit.

It will be appreciated that the function of the high and low voltagebattery management systems 16 and 15 is to monitor and control the stateof charge of the respective battery 18 and 17 to which they areconnected and to prevent over discharging or excessive charging of thesebatteries 18, 17.

With reference to FIG. 2 there is shown a high level flowchart of amethod 100 for starting the engine 11 that could be embodied as softwareor firmware as part of the electronic controller 10 and forms a firstpart of a method for improved engine operation.

The method starts in box 110 with a key-on event and with the engine 11not running, and then advances to box 115 to check whether an enginestart request has been received either due to a manual start operationby a user of the engine 11 or due to an automatic start request at theend of an E-stop.

If no start request has been received the method cycles through box 115until either a key-off event occurs or an engine start request isreceived.

If a key-off event occurs, the method terminates with the engine 11still not running and, if an engine start request is received, themethod advances to box 118.

In box 118 it is checked whether there is sufficient charge in the highvoltage battery 18 to effectively start the engine using the BISG 14and, if there is not, the method advances to box 160 to start the engineusing the starter motor 13 and, if there is sufficient charge in thehigh voltage battery 18, the method continues to box 120.

In box 120 it is checked whether a current engine temperature (T) isabove a minimum temperature limit (T_(min)). As previously discussedthis temperature limit is a positive temperature within a few degrees ofzero degrees Celsius or a negative temperature of up to about minus 20degrees Celsius. The temperature used for this test could be thetemperature of part of the engine 11 such as a cylinder block orcylinder head or the temperature of the engine coolant depending uponthe specific arrangement of the engine system and sensors. In oneexample the temperature limit was set at 0° C. and the measuredtemperature was a measurement of cylinder block temperature.

The temperature limit (T_(min)) therefore falls normally within therange of −40 to +5 degrees Celsius depending upon the torque capacity ofthe BISG 14 and the relationship between the torque required to startthe engine 11 and temperature.

If the current temperature is above the minimum temperature limitT_(min) then the method advances to box 130 otherwise it advances to box160.

In box 130 the engine 11 is cranked using the BISG 14 and the resultingcranking speed (Nc) is measured using a rotational speed sensor. Then inbox 140 it is checked whether the cranking speed is above a speed limitNc_(lim) chosen to represent a speed where a good quality engine startwill be produced if the BISG 14 is operating correctly. The speed is notnecessarily the final speed expected to be reached by the engine 11 whencranked by the BISG 14 it could be a speed expected to be attained bythe engine a predetermined period of time after initiation of cranking.This test is used to confirm that the BISG 14 is operating correctly andis capable of producing a high quality engine start. As previouslyreferred to, this test could be replaced by an engine acceleration test.In which case the test would be replaced in box 140 with:—

Is the measured engine acceleration greater than a required rate ofengine acceleration?

It will be appreciated that the logic used could be reversed so that thetest could check whether the engine speed or engine acceleration is lessthan a defined value and in which case the Values of “Yes” and “No”would need to be reversed.

However, as shown, if Nc is greater than Nc_(lim) then the methodadvances to box 150 otherwise it advances to box 160.

In box 150 it is checked whether the engine 11 has started. This checkcould be performed by measuring the rotational speed of the engine or byany other means such as for example measuring fuel usage or monitoringexhaust gas constituents.

If the engine 11 has started when checked in box 150 the engine startmethod advances to box 190 with the engine running otherwise it returnsfrom box 150 to box 130 to continue cranking the engine 11 using theBISG 14 and cycles through boxes 130 to 150 until the engine 11 hasstarted.

A further step may be included in the return from box 150 to box 130 tocheck that the current SOC of the high voltage battery 18 is above apredefined lower charge limit and if it is not starting using the BISG14 will need to be aborted.

Referring now back to boxes 120 and 140, if these tests are failed themethod advances in both cases to box 160 where the engine 11 is crankedusing the starter motor 13 and then advances to box 170 to check whetherthe engine 11 has started. If the engine has not started the methodcycles through boxes 160 and 170 until it does start and when it doesstart advances to box 190 where the engine start method ends with theengine 11 running

It will be appreciated that the method 100 will be ended at any step ifa key-off event occurs. It will also be appreciated that, if the enginehas not started within a predefined period of time following initiationof cranking, further steps (not shown) may be provided to halt crankingof the engine to prevent damaging either the starter motor 13 or theBISG 14 depending upon which starting device is being used.

FIG. 3 shows a flow chart of a method 200 forming a second part of themethod for improved engine operation.

The method uses the engine starting method 100 shown in FIG. 2 andfollows on from step 190 of that method with the engine running.

In box 210 it is checked whether the temperature of the exhaust gas fromthe engine 11 needs to be increased (exhaust gas heating) in order toassist with light-off of any exhaust aftertreatment devices. This stepcould be carried out by means of a comparison of a current measurementof exhaust temperature using a temperature sensor located in or adjacentto each aftertreatment device 6 with a predefined temperature limit orby any other convenient means including exhaust temperature modelling.

One example of the test in box 210 is

-   Is Texh<Texh_(lim) ? If yes GoTo 220 else GoTo 260-   Where-   Texh is the current measured exhaust gas temperature; and Texh_(lim)    is an exhaust gas temperature above which light-off of the    respective aftertreatment device 6 can be expected to have occurred.    Therefore, assuming that exhaust gas heating is required, the method    advances from box 210 to box 220.

In box 220 it is checked whether the HV battery 18 can be charged.Normally if the BISG 14 has been used to restart or start the engine 11then there will be sufficient headroom (the difference between currentSOC and maximum permitted SOC) in the high voltage battery 18 (HVbattery 18) to permit it to be recharged. However, if the starter motor13 has been used to start the engine 11, then the SOC of the HV battery18 could be too high to permit charging to be carried out.

That is to say, the SOC of the HV battery 18 could be at the maximumcharging limit (fully charged) or so close to this limit that the amountof charging that could be carried out would have no real effect onexhaust gas heating.

If charging is not possible the method in this case advances to box 290where it ends. After reaching box 290 control of the BISG 14 will thenrevert to normal control to meet any needs of the HV battery 18 and lowvoltage battery 17 as determined by the high and low voltage batterymanagement systems 16 and 15 and/or torque assist of the engine 11 ifsuch a facility is permitted and is required.

Torque assist is the use of the BISG 14 to provide torque to assist theengine 11. That is to say for a given torque requirement some of thetorque is provided by the BISG 14 thereby reducing the amount of torquerequired from the engine 11 and resulting in improved fuel economy.

It will be appreciated that the BISG 14 could also or alternatively beused for ‘torque boosting’. Torque boosting is where the BISG 14 is usedto increase the torque supplied to the vehicle above that availablesolely from the engine 11. This will not reduce the fuel consumption ofthe engine 11 but instead will increase vehicle performance.

If in box 220 it is confirmed that there is sufficient headroom in theHV battery 18 for charging to occur then the method advances from box220 to box 230 and the BISG 14 is operated in the first mode as agenerator to load the engine 11 thereby increasing the exhaust gastemperature exiting the engine 11. In such a case the BISG 14 isoperated at or close to its maximum output level in order to produce themaximum loading on the engine 11.

From box 230 the method advances to box 240 which in practice occurssimultaneously with the action performed in box 230. That is to say, theSOC of the HV battery 18 is continuously monitored to preventovercharging of the HV battery 18. If the SOC of the HV battery 18 isbelow the maximum charging limit then charging can continue but it isfirst checked whether exhaust gas heating is still required and so themethod returns to box 210. The method will then proceed through boxes220 and 230 back to box 240 provided heating is still required and willcontinue to cycle through boxes 210 to 240 until either the maximumcharging limit of the HV battery 18 is reached or heating of the exhaustgasses is no longer required.

If the maximum charging limit is reached then the method advances frombox 240 to box 250 where charging of the HV battery by the BISG 14 isterminated and will then advance to box 290 where it will end and, aspreviously described, the control of the BISG 14 will revert to normalcontrol to meet any needs of the HV battery 18.

If exhaust gas heating is no longer required the method will advancefrom box 210 to box 260.

If torque assist or torque boosting is not a function available on thevehicle 5 then the method would instead advance directly from box 210 tobox 290 and boxes 260 to 280 would not be present.

In box 260 it is checked whether torque assist is required and, if it isnot, the method advances from box 260 to box 290 where it ends. It willbe appreciated that torque assist is not permitted if the currentexhaust gas temperature is below the exhaust gas temperature limitTexh_(lim) below which light-off of the respective aftertreatment can beexpected not to have occurred. This is because torque assist will havethe effect of reducing exhaust gas temperature not increasing it and sowill delay light-off of the aftertreatment devices 6.

If torque assist is required, and permitted, the method advances frombox 260 to box 270 where the current SOC of the HV battery 18 ischecked. The SOC of the HV battery 18 is not allowed to fall below alower charge limit and so before permitting torque assist is allowed itis checked to ensure that a sufficient amount of charge is present inthe HV battery 18 to produce a worthwhile torque assist effect.

If there is insufficient charge in the HV battery 18 to permit torqueassist, the method advances from box 270 to box 290 where it ends.However, if in box 270 it is confirmed that sufficient charge isavailable to permit torque assist, the method advances from box 270 tobox 280 where the BISG 14 is operated as a motor to provide assistanceto the engine 11.

From box 280 the method returns to box 260 and will then continue tocycle through boxes 260, 270 and 280 until either the SOC of the HVbattery 18 falls to a level below which torque assist is not permitted(box 270 fail) or torque assist is no longer required (box 260 fail). Ineither of these cases the method will advance from the respective box tobox 290 where it ends and, as previously described, the control of theBISG 14 will then revert to normal control to meet any needs of the HVbattery 18.

Therefore in summary the belt integrated starter motor is preferablyused to start the engine because this will result in a discharging ofthe associated high voltage battery which can then be rapidly rechargedfollowing an engine start to aid with heating of the exhaust gasses andreduce the time required for any exhaust aftertreatment device to reachtheir respective light-off temperatures. The starter motor is only usedif the BISG is not capable of producing an effective start or thetemperature is so low that use of the BISG is not possible.

By giving preference to starting of the engine using the BISG wheneverpossible ensures that the BISG can be used, if required, immediatelyfollowing the start as a generator load for the engine so as to assistwith exhaust gas heating.

Some advantages may include

-   a/ Conducting an engine start using the 48V BISG, consumes energy    from the 48V battery and then immediately after cranking ends the    48V BISG can be used to re-charge the 48V battery and so load the    engine. This engine loading will generate more exhaust heat during a    cold start period following start-up and will result in faster    exhaust gas aftertreatment light-off than would otherwise be    possible;-   b/ The faster light-off will result in lower engine emissions which    will allow the use of a Mild Hybrid control strategy that is more    focused on CO2 reduction without exceeding emission targets for the    engine;-   c/ When compared to the cranking attributes of a 12V starter, the    driver in the vast majority of cases will get a higher start quality    for the first engine start due to the use of the 48V BISG. The 12V    starter is therefore only used when the ambient temperature is low    such as less than about 4° C. or 48V BISG system is unable to crank    effectively;-   d/ Using the BISG to start the engine produces a better quality    start in terms of noise and vibration than a starter motor and less    mechanical wear in terms of the components required to effect the    start; and-   e/ Providing a larger range of operation for the BISG increases the    possibilities for stop-start operation. That is to say, E-stop can    be performed even if the engine has not reached it normal operating    temperature range.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention.

What is claimed is:
 1. A method comprising: starting an engine of avehicle having a starter motor and a belt integrated starter-generator(BISG) outside a torque path between the engine and wheels using onlythe BISG if engine temperature is greater than a predefined temperaturelimit; and if after the starting it is required to activate an exhaustafter treatment device, using the BISG as a generator to load the engineto increase engine exhaust gas temperature.
 2. The method as claimed inclaim 1 further comprising using the BISG as a generator to increase theengine exhaust gas temperature until it is no longer required toincrease the engine exhaust gas temperature.
 3. The method as claimed inclaim 1 further comprising using the BISG as a generator only if a stateof charge of a battery charged by the BISG is less than a predefinedupper charging limit.
 4. The method as claimed in claim 1 furthercomprising, if after the starting it is not required to activate theexhaust after treatment device, using the BISG as a motor to providetorque assist to the engine if torque assist is required.
 5. The methodas claimed in claim 4 further comprising using the BISG as a motor toprovide torque assist only if a state of charge of a battery providingpower to the BISG is greater than a minimum permitted charge level. 6.The method as claimed in claim 1, wherein the predefined temperaturelimit falls within a range of −40 to +5 degrees Celsius.
 7. A methodcomprising: starting an engine of a vehicle having a starter motor and abelt integrated starter-generator (BISG) outside a torque path betweenthe engine and wheels using only the BISG to reduce a state of charge(SOC) of a battery used to power the BISG; and if after the starting itis required to activate an exhaust after treatment device and the SOC isless than a predefined level, using the BISG as a generator to load theengine to increase engine exhaust gas temperature.
 8. The method asclaimed in claim 7 further comprising using the BISG as a generator toincrease the engine exhaust gas temperature until it is no longerrequired to increase the engine exhaust gas temperature.
 9. The methodas claimed in claim 7 further comprising, if after the starting it isnot required to activate the exhaust after treatment device, using theBISG as a motor to provide torque assist to the engine if torque assistis required.
 10. The method as claimed in claim 7 further comprisingusing the BISG as a motor to provide torque assist only if the SOC isgreater than a minimum permitted charge level.